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Summary When the bursa of Fabricius was isolated from environmental stimuli across the bursal epielium by clipping the bursal duct on the 19th day of incubation, marked retardation was observed both in the bursal development and in the maturation of the immune reactivity. Neither necrotic changes nor involution in the bursa was, however, observed at any stage examined after clipping the bursal duct. Lymphopoiesis in the bursa and the immune reactivity of these chickens gradually increased with age, although they were constantly lower than those of controls. These results suggest that environmental stimuli across the bursal epithelium play an important role in the amplification mechanisms for B‐cell differentiation in the bursa.

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